Bump Scala 2.11 version, SBT version, etc.

This commit upgrades the following versions:

 * Scala 2.11.2 -> 2.11.4
 * SBT 0.13.5 -> 0.13.7
 * ScalaTest 2.1.3 -> 2.2.1
 * ScalaCheck 1.11.3 -> 1.11.6